The invention relates to a device for the cooling of bulk material by low-boiling, liquefied gas in a precooling zone and in a spray zone. Such bulk material is scrap metal, for example, which is fed into a shredder. Such material may also be scrap tires or spices which are fed into a mill or may be rubber and plastic scraps.
The cooling of such bulk goods frequently takes place in cooling tunnels. Here, the bulk goods are poured onto a conveyer belt of steel and conducted through an insulated tunnel which is divided into a precooling zone and a spraying zone. The coolant, as a rule, is liquid nitrogen. In the precooling zone, cold nitrogen, vaporized on the conveyor belt, encounters the bulk goods. The bulk goods are cooled down thereby and reach the spraying zone at an already relatively low temperature. In the spraying zone, liquid nitrogen is sprayed on the goods, whereby they are cooled to the desired low temperature.
The use of such cooling tunnels for cooling bulk goods is relatively expensive. In particular the conveyer belt of austenitic steel is expensive.
In addition, prior practices have the drawback that, depending on the design, 20-30% of the heat of vaporization of the liquid nitrogen is transported into the warmth. Since the conveyer belts cannot be lubricated because of the low temperatures, they are subject to extensive wear, particularly in the joints. The cooling on the conveyor belt is uneven, furthermore, since the material, lying directly on the conveyer belt, remains warmer than that lying on top. Conveyer belts are indeed indispensable for sensitive goods, e.g. many foodstuffs, since mechanical damage to the goods by impacts and jolts is avoided. For crude goods, however, which may be poured, conveyer belts are too expensive.
A further area with which this invention is concerned is scrap tires. The disposal of the many millions of old tires accumulating each year presents an increasingly more difficult problem. Earlier, the old tires were stored in mill deposits. Because of the large quantity, this is possible only to a small extent. Furthermore, tires have a very long decomposition time. The hollow spaces resulting in storage have many disadvantages. These may be avoided if scrap tires are rendered into small pieces, the disintegration of the elastic material, however, already presenting considerable difficulties.
The burning of old tires is also difficult; however, in specially constructed installations, old tires may be burned satisfactorily. However, it is disadvantageous that the valuable tire material is completely destroyed.
A further possibility of destroying old tires consists of embrittling the old tires by a liquefied, low-boiling gas and subsequently to disintegrate them. For reasons of cost, nitrogen is considered above all as the low-boiling gas. The embrittled tires may relatively easily be rendered into small fragments or grains.
The steel inserts may then subsequently easily be removed. The valuable raw products are retained and may be further employed for various purposes.
In such practices the cooling takes place in that the tires are dipped into a bath of liquid nitrogen. It is disadvantageous here that a large quantity of nitorgen is used up, since in removing the tires from the nitrogen bath, liquid nitrogen remains in the hollow spaces in the tires and is lost, the nitrogen per se not being required for the embrittling of tires. A further disadvantage is the slow rate of travel in such baths. For an economical process, throughput quantities of at least 1000 pieces per hour are strived for, something which may be achieved by parallel-connecting several baths. This, however, is equivalent to a large investment of capital.